The present invention relates to a polishing techniques for the planarization of a surface of a wafer and, more specifically, to a semiconductor device fabricating method which planarizes a thin film formed on a semiconductor wafer by polishing.
The number of components per semiconductor IC (integrated circuit) device has progressively increased in recent years and the component elements of semiconductor IC devices have been miniaturized accordingly. When forming those elements of a semiconductor IC device, films need to be patterned by lithography techniques. Light of a short wavelength and an optical element having a large numerical aperture must be used to form a minute pattern, which, however, involves reduction in the focal depth of the optical element. A semiconductor device fabricating method comprises many pattern forming processes. A metallization process will be described with reference to FIGS. 2A to 2F by way of example.
FIG. 2A is a sectional view of a semiconductor wafer 1 of silicon provided with a first interconnection layer 3 of aluminum thereon. A insulating film 2 is formed on a surface of the wafer 1 so as to cover transistors formed on the wafer 1. The first interconnection layer 3 is formed on the first insulating film 2. Parts 3xe2x80x2 of the first interconnection layer 3 corresponding to holes formed in the insulating film 2 are recessed. An interlayer dielectric film 4, i.e., a silicon dioxide film inmost cases, and a second wiring line 5 of aluminum are formed on the first interconnection layer 3. A photoresist is applied to the second interconnection layer 5 to form a photoresist film 6 to be used for patterning the second interconnection layer 5 as shown in FIG. 2B.
Subsequently, as shown in FIG. 2C, the photoresist film 6 is exposed to light by a stepper 7 provided with a photomask and a demagnification projection lens to form an image of a circuit pattern formed on the photomask. If the stepper 7 has a small focal depth, the image of the circuit pattern cannot be focused properly on both recessed parts and projecting parts 8 of the photoresist film 6 and the image cannot be formed in a satisfactory resolution. Generally, the surface of the wafer 1 provided with the transistors is irregular, the stepper 7 must have a great focal depth.
The surface of the interlayer dielectric film 4 is subjected to a planarizing process to solve the foregoing problems. As shown in FIG. 2D, the interlayer dielectric film 4 is formed on the first interconnection layer 3 formed as shown in FIG. 2A. Portions of the interlayer dielectric film 4 projecting from a level 9 below the bottoms of the recessed parts of the interlayer dielectric film 4 are removed by the planarizing process using a chemical mechanical polishing method (hereinafter referred to as xe2x80x9cCMP methodxe2x80x9d) to planarize the surface of the interlayer dielectric film 4 as shown in FIG. 2E. Then, the second interconnection layer 5 of aluminum and the photoresist film 6 are formed to be exposed to light by the stepper 7. Since the surface of the photoresist film 6 thus formed is flat, the image of the circuit pattern can be formed in a satisfactory resolution even if the focal depth of the stepper 7 is small.
The CMP method is disclosed in U.S. Pat. No. 4,944,836 and Japanese Examined Patent Publication No. Hei 5-30052. FIG. 3 is a typical view of a CMP device illustrating the conception of the CMP method. As shown in FIG. 3, a circular polishing pad 11 is stuck on a turntable 12. The turntable 12 is rotated, for example, in a counterclockwise direction. The polishing pad 11 is, for example, a thin sheet of foam urethane resin formed by slicing a foam urethane resin block. Polishing pads respectively having different qualities and different degrees of minuteness in surface structure are used selectively according to the type of workpieces and desired surface roughness in which the surfaces of workpieces are to be finished. The wafer 1 is fixed to an elastic backing pad 13 fixed to a wafer holder 14. The wafer holder 14 is rotated in the same direction as the turntable 12, the wafer 1 is pressed against the surface of the polishing pad 11, and a polishing slurry 15 containing abrasive powder is supplied onto the polishing pad 11 to planarize the surface of the wafer 1 by polishing.
When polishing an insulating film of silicon dioxide or the like, high-purity silica (fumed silica) powder is used for polishing. The grain size of the silica powder is in the range of about 30 to about 150 nm. The polishing slurry 15 is prepared by suspending silica particles in an alkaline solution, such as a potassium hydroxide solution or an ammonia solution. The polishing slurry 15 is able to finish the surface of the wafer 1 in a flat, smooth surface not damaged significantly.
Another planarizing technique uses a fixed abrasive tool instead of the polishing slurry. A polishing device employed in executing the planarizing technique using the fixed abrasive tool has the same construction as the polishing device employed in the CMP technique, except that the former uses a fixed abrasive tool instead of the polishing pad. The fixed abrasive tool is attached to a platen, and deionized water is supplied instead of the abrasive slurry onto the fixed abrasive tool. This planarizing technique employing the fixed abrasive tool is disclosed in PCT International Publication No. WO 97/10613 and Japanese Patent Laid-open No. Hei 8-216023.
Pattern size dependence is a generally used quantitative index of macroscopic planarizing ability. When a wafer provided with a large pattern and a small pattern is subjected to polishing, the small pattern is polished at a polishing rate higher than that at which the large pattern is polished. When a large pattern and a small pattern are polished by a polishing process having a low planarizing ability, the difference in abraded amount between the large pattern and the small pattern is large. More concretely, pattern size dependence can be determined by polishing a pattern having some isolated lines having a height of about 0.8 xcexcm and widths in the range of about 0.1 xcexcm to about 5 mm, and spaces between the lines formed on a test wafer as shown in FIG. 8, and measuring the differences between the amounts of abraded portions of the isolated lines. When narrows lines of widths less than 1 mm and wide lines of widths not smaller than 3 mm are polished by using a standard polishing pad and a standard polishing slurry, the height of the wide lines is 0.38 xcexcm or above when the narrow lines are abraded completely, and the wide lines cannot completely be abraded even if the polishing process is continued further.
As mentioned above, it is difficult to planarize completely a layer formed over a pattern and having steps corresponding to the pattern by the conventional CMP process. However, a memory mat of an actual 64 Mbit DRAM (dynamic random-access memory) has 8 to 10 mm sq. pattern elements of about 0.8 xcexcm in height.
The foregoing planarizing process employing the fixed abrasive tool has an excellent ability to planarize pattern elements including large ones. The planarizing process employing the fixed abrasive tool is characterized by its very high planarizing ability. For example, as obvious from FIG. 8, whereas the CMP process planarizes a 3 mm wide pattern element to a height of 0.38 xcexcm (380 nm), the planarizing process using the fixed abrasive tool is able to planarize the same pattern element to a height of 18 nm, which is extraordinarily small as compared with 380 nm.
The planarizing process employing the fixed abrasive tool is able to planarize pattern elements of large sizes of several millimeters or above which cannot satisfactorily be planarized by the CMP process because the abrasive grains of the fixed abrasive tool are fixed and the fixed abrasive tool has a high elastic modulus. Furthermore, since the fixed abrasive tool is scarcely subject to deformation, only projections on the surface of a workpiece can selectively be removed. Unlike the CMP process in which the polishing pad deforms conforming to the irregularities in the surface of the workpiece, the polishing process employing the fixed abrasive tool does not cut the bottoms of recessed parts in the surface of the workpiece. Therefore, when polishing the workpiece by the polishing process employing the fixed abrasive tool, it is unnecessary to take into account the amount of abrasion of recessed parts when estimating the amount of abrasion necessary for planarization, and hence the film to be planarized may be formed in a relatively small thickness. Since the fixed abrasive tool has abrasive grains, any polishing slurry is not necessary, the polishing apparatus for carrying out the polishing process employing the fixed abrasive tool needs very small maintenance costs.
Although the polishing process employing a fixed abrasive tool has an excellent planarizing ability, it liable to form scratches in the polished surface. The inventors of the present invention found that scratches can be classified into those of a macro scratch group and those of a micro scratch group. Macro scratches of the macro scratch group penetrate the interlayer dielectric film and have length in the range of 5 xcexcm to several millimeters. Micro scratches of the micro scratch group do not penetrate the interlayer dielectric film, and have depth of 100 nm or below and length of 10 xcexcm or below.
If scratches are formed in the interlayer dielectric film, metal films formed in the scratches cannot be removed by planarization to be executed in a Damascene metallization process and remains in the surface of the interlayer dielectric film as shown in FIG. 10. It is possible that metal films thus formed in micro scratches short-circuit adjacent wiring lines formed in the interlayer dielectric film. It is possible that metal films formed in macro scratches are connected to wiring lines underlying the interlayer dielectric film and short-circuit the wiring lines overlying the interlayer dielectric film and those underlying the interlayer dielectric film.
If scratches are formed in active regions in which gates are to be formed during a planarizing process for planarizing a film formed in shallow trenches for shallow trench isolation, defects are produced in silicon crystals to deteriorate the characteristics of transistors formed in the active regions.
It is a first object of the present invention to provide a planarizing method employing a fixed abrasive tool capable of polishing a surface without forming detrimental scratches. A second object of the present invention is to provide a planarizing method capable of reducing micro scratches or preventing the formation of micro scratches.
A third object of the present invention is to provide a semiconductor device fabricating method capable of producing semiconductor devices at low costs.
A fourth object of the present invention is to provide a semiconductor device fabricating method capable of preventing short circuit between wiring lines.
A fifth object of the present invention is to provide a semiconductor device fabricating method capable of fabricating semiconductor devices provided with highly reliable component elements.
The above objects are attained by using a fixed abrasive tool containing impurities harder than a workpiece in an impurity content of 10 ppm or below. The present invention can more effectively be achieved by using a fixed abrasive tool containing lanthanum (La) in a lanthanum content of 10 ppm or below if the fixed abrasive tool is composed of abrasive grains of cerium dioxide (CeO2). FIG. 4 is a table showing the impurity contents of a conventional abrasive and a purified abrasive. Cerium dioxide produced by purifying natural rocks unavoidably contains hard lanthanum dioxide.
Elimination of lanthanum from cerium dioxide is effective in reducing scratches. Hard impurity content of cerium dioxide abrasive can effectively be reduced by increasing the purity of cerium dioxide and reducing lanthanum content, and margin for forming scratches can be enlarged.
FIG. 5 is a graph showing the scratching characteristics of fixed abrasive tools respectively having different lanthanum contents (1300 ppm and 9 ppm) determined by polishing insulating films. As obvious from FIG. 5, macro scratch forming frequency can be reduced to {fraction (1/10)} when the impurity content is reduced. The hardness of each substance can be expressed in Moh""s hardness. For example, silicon dioxide (SiO2), cerium dioxide (CeO2), alumina (Al2O3) and iron oxide (Fe2O3) are 6.75, 6, 9 and 6.75 in Moh""s hardness, respectively.
The objects of the present invention can effectively be achieved by using needle abrasive grains shaped with breadth diameter and length diameter (or rice-shaped abrasive grains). It is known from FIG. 5 that the use of needle abrasive grains is effective in reducing macro scratches. The use of needle abrasive grains having length diameter/breadth diameter rations of 3 or above is particularly effective and is effective in reducing micro scratches. Abrasives having a 10% wt. needle grain content may be used. It is desirable that abrasives have an needle grain content of 50% wt. or above.
The objects of the present invention can be achieved by using a fixed abrasive tool having a mean pore diameter of 0.2 xcexcm or below. A fixed abrasive tool of needle abrasive grains has a low aggregation and has a uniform pore diameter distribution.
Generally, a method of manufacturing fixed abrasive tools comprises the steps of (1) kneading a mixture of abrasive grains and resin grains, (2) forming, (3) heating and compression molding and (4) molding removal. A fixed abrasive tool has pores (pores indicated at 19 in FIG. 1). The porosity of the fixed abrasive tool is adjusted by amount of compression used in the step (3). Usually, the porosity of fixed abrasive tool is about 50%. Porosity is not adjusted when manufacturing a conventional fixed abrasive tool, and the conventional fixed abrasive tool has a mean pore diameter of about 0.3 xcexcm and pore diameters are distributed in a wide range as shown in FIG. 7.
Since needle abrasive grains have a low aggregating property owing to their shape, needle abrasive grains form a fixed abrasive tool having pores of pore diameters distributed in a narrow range and having a mean pore diameter on the order of 0.1 xcexcm. The reduction of the maximum pore diameter to 0.5 xcexcm or below is highly effective in reducing macro scratches and also effective in reducing micro scratches. Although it is desirable that pores of the fixed abrasive tool do not include pores of diameters not smaller than 0.5 xcexcm, there is no practical problem in using the fixed abrasive tool having pores of diameters not smaller than 0.5 xcexcm, provided that the content of such large pores is 10% vol. or below. The pore diameter distribution in the fixed abrasive tool was measured by a mercury porosimeter.
The objects of the present invention can be achieved by using abrasive grains of 1 g/cm3 or below in bulk density. Bulk density is the ratio of the weight of a fixed volume of abrasive grains to the fixed volume. The bulk density decreases as the pores increase. The bulk density is large when the number of pores is small and abrasive grains are packed densely.
The respective bulk densities of polyhedral abrasive grains forming a conventional fixed abrasive tool and needle abrasive grains forming a fixed abrasive tool employed in the present invention are 2 g/cm3 and 0.5 g/cm3. Needle abrasive grains have a small bulk density and hence the porosity of a fixed abrasive tool formed of needle abrasive grains can be adjusted properly in a wide porosity range. It is known that the length diameter-to-breadth diameter ratio of needle abrasive grains is 3 or above. Such shapes of needle abrasive grains makes the bulk density of the needle abrasive grains small.
The object of the present invention can be achieved by using abrasive grains having a maximum grain size of 1 xcexcm or below. Use of abrasive grains having small grain sizes suppresses stress concentration on the surface of a workpiece and hence reduces micro scratches. As obvious from FIG. 6, the number of micro scratches increases substantially in proportion to grain size as shown in FIG. 6.
The finer the abrasive grains, the greater is the effect in reducing micro scratches. However, abrasive grains having a submicron mean grain size aggregate unavoidably and the apparent grain size of the abrasive grains increases. Abrasive grains having a mean grain size not greater than 0.5 xcexcm are practically effective in reducing micro scratches and, if the maximum grain size of the abrasive grains is not greater than 1 xcexcm, the formation of micro scratches can more effectively prevented. Although it is desirable to use abrasive grains not including those of grain sizes not smaller than 1 xcexcm, abrasive grains containing abrasive grains of grain sizes not smaller than 1 xcexcm in a content of 1% wt. or below are practically acceptable.
The grain size distribution can be measured by a grain size distribution measuring apparatus that detects scattered laser light, such as a particle size analyzer commercially available from HORIBA or a particle size analyzer SALD-2000A commercially available from Shimazu Corp. A method of determining grain size distribution measures the sizes of images of abrasive grains on magnified photographs of abrasive grains, which, however, has difficulty in increasing the number of samples and hence is unable to provide accurate data. When measuring grain size distribution, it is important to examine measuring conditions thoroughly, to determine optimum conditions for a method of stirring samples taking into account the aggregating property of the abrasive grains and to confirm the reproducibility by repeating measurement at least three times.
The application of the present invention to processing an insulating film and a metal film reduces macro and micro scratches in the insulating film and the metal film. Thus, the present invention provides a reliable semiconductor IC device fabricating method. The fixed abrasive tool may be of multilayer structure. When the fixed abrasive tool is of multilayer structure, it is necessary that only one of the layers of the fixed abrasive tool to be brought into contact with a workpiece meets the foregoing requirements.
These and other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.